I've used 3 and 5 gallon bags without adding any thing like pebbles for drainage level. Next season, I'm moving up to larger bags and may include fertilizer in my drip lines. Grow bags and timed drippers gave me great results with a minimum of effort. Just plant, run the irrigation, feed and harvest. From my experience, the pebbles are unnecessary.

Thank you for your response!

This will be my first experience with the grow bags. I will just be using potting mix though, no hydroponic growing here yet for this newbie. LOL!

Thanks again!

My grow bags were potting mix too. I just used an automatic system to water every day. It saved me so much time. Another reason to skip the rocks; after a season or two and the soil is all spent it's a paint to sort rocks from roots and soil. I just dumped a few older containers and the ones with rocks in the bottom were a real mess.